Smoke sim extremely pixely/blocky
The Domain Resolution is the key here since it defines how large the cells of the fluid simulation are - and it is those cells that you are seeing as the ‘blockiness’.
Is is important to Apply Scale to ensure the dimensions of the cells are regular (otherwise the simulation will be skewed) but then the size of the cells it determined simply by dividing the longest dimension (width, height, depth) of the domain by the Resolution - so, for example, doubling the Resolution will result in twice as many cells in each dimension.
If your previous simulation was Baked the you’ll need to Free Bake to allow the simulation to be re-baked with the new settings.
Note that doubling the resolution will result in 2x2x2 = 8 times the number of cells in the simulation (so 8x the cpu effort to simulate and 8 times the memory overhead) but a more accurate and detailed simulation.